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Chapter 3: Relational Model
 Structure of Relational Databases
 Relational Algebra
 Tuple Relational Calculus
 Domain Relational Calculus
 Extended Relational-Algebra-Operations
 Modification of the Database
 Views
Database System Concepts
3.1
©Silberschatz, Korth and Sudarshan
Example of a Relation
Database System Concepts
3.2
©Silberschatz, Korth and Sudarshan
About Relational Model
Order of tuples not important
Order of attributes not important (in theory)
Collection of relation schemas (intension)
Relational database schema
Corresponding relation instances (extension)
Relational database
intension vs. extension
schema vs. data
metadata
includes schema
Database System Concepts
3.3
©Silberschatz, Korth and Sudarshan
Why Relations?
 Very simple model.
 Often a good match for the way we think about our data.
 Abstract model that underlies SQL, the most important language
in DBMS’s today.
 But SQL uses “bags” while the abstract relational model is setoriented.
Database System Concepts
3.4
©Silberschatz, Korth and Sudarshan
Relational Design
Simplest approach (not always best): convert each E.S. to a relation and each
relationship to a relation.
Entity Set  Relation
E.S. attributes become relational attributes.
name
manf
Becomes:
Beers(name, manf)
Database System Concepts
Beers
3.5
©Silberschatz, Korth and Sudarshan
Relational Model

Table = relation.

Column headers = attributes.

Row = tuple

name
manf
WinterBrew Pete’s
BudLite Beers A.B.
…
…
Relation schema = name(attributes) + other structure info.,
e.g., keys, other constraints. Example: Beers(name, manf)
 Order of attributes is arbitrary, but in practice we need to assume the order given in the relation
schema.

Relation instance is current set of rows for a relation schema.

Database schema = collection of relation schemas.
Database System Concepts
3.6
©Silberschatz, Korth and Sudarshan
Basic Structure
 Formally, given sets D1, D2, …. Dn a relation r is a subset of
D1 x D2 x … x Dn
Thus a relation is a set of n-tuples (a1, a2, …, an) where
each ai  Di
 Example: if
customer-name = {Jones, Smith, Curry, Lindsay}
customer-street = {Main, North, Park}
customer-city = {Harrison, Rye, Pittsfield}
Then r = { (Jones, Main, Harrison),
(Smith, North, Rye),
(Curry, North, Rye),
(Lindsay, Park, Pittsfield)}
is a relation over customer-name x customer-street x customer-city
Database System Concepts
3.7
©Silberschatz, Korth and Sudarshan
Relational Data Model
Relation as table
Rows = tuples
Columns = components
Names of columns = attributes
Set of attribute names = schema
REL (A1,A2,...,An)
A1 A2 A3 ... An
C
a
r
d
i
n
a
l
i
t
y
a1 a2 a3
an
b1 b2 a3
cn
a1 c3 b3
.
.
.
bn
x1 v2 d3
wn
Attributes
Tuple
Set theoretic
Domain — set of values
like a data type
Cartesian product (or product)
D1 D2 ...  Dn
n-tuples (V1,V2,...,Vn)
s.t., V1 D1, V2 D2,...,Vn Dn
Relation-subset of cartesian product
of one or more domains
FINITE only; empty set allowed
Tuples = members of a relation inst.
Arity = number of domains
Components = values in a tuple
Domains — corresp. with attributes
Cardinality = number of tuples
Component
Arity
Database System Concepts
3.8
©Silberschatz, Korth and Sudarshan
Relation: Example
Name
address
tel #
5
3
7
Domain of
Relation
Cardinality of domain
N
Domains
N1 A1 T1
A
T
N
A
T
N1
A1
T1
N2
A2
T2
N1 A1 T3
N3
A3
T3
.
N4
T4
.
N5
T5
.
T6
N1 A1 T7
T7
Attribute
Database System Concepts
3
Cardinality
<=5x3x7
of relation
N1 A1 T2
Component
N1 A2 T1
N1 A3 T1
Domain
Arity
Tuple µ
N2 A1 T1
3.9
©Silberschatz, Korth and Sudarshan
Attribute Types
 Each attribute of a relation has a name
 The set of allowed values for each attribute is called the domain
of the attribute
 Attribute values are (normally) required to be atomic, that is,
indivisible
 E.g. multivalued attribute values are not atomic
 E.g. composite attribute values are not atomic
 The special value null is a member of every domain
 The null value causes complications in the definition of many
operations
 we shall ignore the effect of null values in our main presentation
and consider their effect later
Database System Concepts
3.10
©Silberschatz, Korth and Sudarshan
Relation Schema
 A1, A2, …, An are attributes
 R = (A1, A2, …, An ) is a relation schema
E.g. Customer-schema =
(customer-name, customer-street, customer-city)
 r(R) is a relation on the relation schema R
E.g.
Database System Concepts
customer (Customer-schema)
3.11
©Silberschatz, Korth and Sudarshan
Relation Instance
 The current values (relation instance) of a relation are
specified by a table
 An element t of r is a tuple, represented by a row in a table
attributes
(or columns)
customer-name customer-street
Jones
Smith
Curry
Lindsay
Main
North
North
Park
customer-city
Harrison
Rye
Rye
Pittsfield
tuples
(or rows)
customer
Database System Concepts
3.12
©Silberschatz, Korth and Sudarshan
Relation Instance
Name
Address
Bob
123 Main St
555-1234
Bob
128 Main St
555-1235
Pat
123 Main St
555-1235
Harry
456 Main St
555-2221
Sally
456 Main St
555-2221
Sally
456 Main St
555-2223
Pat
12 State St
555-1235
Database System Concepts
Telephone
3.13
©Silberschatz, Korth and Sudarshan
Relations are Unordered
 Order of tuples is irrelevant (tuples may be stored in an arbitrary order)
 E.g. account relation with unordered tuples
Database System Concepts
3.14
©Silberschatz, Korth and Sudarshan
Database
 A database consists of multiple relations
 Information about an enterprise is broken up into parts, with each
relation storing one part of the information
E.g.: account : stores information about accounts
depositor : stores information about which customer
owns which account
customer : stores information about customers
 Storing all information as a single relation such as
bank(account-number, balance, customer-name, ..)
results in
 repetition of information (e.g. two customers own an account)
 the need for null values (e.g. represent a customer without an
account)
 Normalization theory (Chapter 7) deals with how to design
relational schemas
Database System Concepts
3.15
©Silberschatz, Korth and Sudarshan
The customer Relation
Database System Concepts
3.16
©Silberschatz, Korth and Sudarshan
The depositor Relation
Database System Concepts
3.17
©Silberschatz, Korth and Sudarshan
E-R Diagram for the Banking Enterprise
Database System Concepts
3.18
©Silberschatz, Korth and Sudarshan
Keys
 Let K  R
 K is a superkey of R if values for K are sufficient to identify a
unique tuple of each possible relation r(R)
 by “possible r” we mean a relation r that could exist in the enterprise
we are modeling.
 Example: {customer-name, customer-street} and
{customer-name}
are both superkeys of Customer, if no two customers can possibly
have the same name.
 K is a candidate key if K is minimal
Example: {customer-name} is a candidate key for Customer,
since it is a superkey (assuming no two customers can possibly
have the same name), and no subset of it is a superkey.
Database System Concepts
3.19
©Silberschatz, Korth and Sudarshan
Determining Keys from E-R Sets
 Strong entity set. The primary key of the entity set becomes
the primary key of the relation.
 Weak entity set. The primary key of the relation consists of the
union of the primary key of the strong entity set and the
discriminator of the weak entity set.
 Relationship set. The union of the primary keys of the related
entity sets becomes a super key of the relation.
 For binary many-to-one relationship sets, the primary key of the
“many” entity set becomes the relation’s primary key.
 For one-to-one relationship sets, the relation’s primary key can be
that of either entity set.
 For many-to-many relationship sets, the union of the primary keys
becomes the relation’s primary key
Database System Concepts
3.20
©Silberschatz, Korth and Sudarshan
Schema Diagram for the Banking Enterprise
Database System Concepts
3.21
©Silberschatz, Korth and Sudarshan
Query Languages
 Language in which user requests information from the database.
 Categories of languages
 procedural
 non-procedural
 “Pure” languages:
 Relational Algebra
 Tuple Relational Calculus
 Domain Relational Calculus
 Pure languages form underlying basis of query languages that
people use.
Database System Concepts
3.22
©Silberschatz, Korth and Sudarshan
Relational Algebra
 Procedural language
 Six basic operators
 select
 project
 union
 set difference
 Cartesian product
 rename
 The operators take two or more relations as inputs and give a
new relation as a result.
Database System Concepts
3.23
©Silberschatz, Korth and Sudarshan
Select Operation – Example
• Relation r
• A=B ^ D > 5 (r)
Database System Concepts
A
B
C
D


1
7


5
7


12
3


23 10
A
B
C
D


1
7


23 10
3.24
©Silberschatz, Korth and Sudarshan
Select Operation
 Notation:
 p(r)
 p is called the selection predicate
 Defined as:
p(r) = {t | t  r and p(t)}
Where p is a formula in propositional calculus consisting
of terms connected by :  (and),  (or),  (not)
Each term is one of:
<attribute> op <attribute> or <constant>
where op is one of: =, , >, . <. 
 Example of selection:
 branch-name=“Perryridge”(account)
Database System Concepts
3.25
©Silberschatz, Korth and Sudarshan
Project Operation – Example
 Relation r:
 A,C (r)
Database System Concepts
A
B
C

10
1

20
1

30
1

40
2
A
C
A
C

1

1

1

1

1

2

2
=
3.26
©Silberschatz, Korth and Sudarshan
Project Operation
 Notation:
A1, A2, …, Ak (r)
where A1, A2 are attribute names and r is a relation name.
 The result is defined as the relation of k columns obtained by
erasing the columns that are not listed
 Duplicate rows removed from result, since relations are sets
 E.g. To eliminate the branch-name attribute of account
account-number, balance (account)
Database System Concepts
3.27
©Silberschatz, Korth and Sudarshan
Union Operation – Example
 Relations r, s:
A
B
A
B

1

2

2

3

1
s
r
r  s:
Database System Concepts
A
B

1

2

1

3
3.28
©Silberschatz, Korth and Sudarshan
Union Operation
 Notation: r  s
 Defined as:
r  s = {t | t  r or t  s}
 For r  s to be valid.
1. r, s must have the same arity (same number of attributes)
2. The attribute domains must be compatible (e.g., 2nd column
of r deals with the same type of values as does the 2nd
column of s)
 E.g. to find all customers with either an account or a loan
customer-name (depositor)  customer-name (borrower)
Database System Concepts
3.29
©Silberschatz, Korth and Sudarshan
Set Difference Operation – Example
 Relations r, s:
A
B
A
B

1

2

2

3

1
s
r
r – s:
Database System Concepts
A
B

1

1
3.30
©Silberschatz, Korth and Sudarshan
Set Difference Operation
 Notation r – s
 Defined as:
r – s = {t | t  r and t  s}
 Set differences must be taken between compatible relations.
 r and s must have the same arity
 attribute domains of r and s must be compatible
Database System Concepts
3.31
©Silberschatz, Korth and Sudarshan
Cartesian-Product Operation-Example
Relations r, s:
A
B
C
D
E

1

2




10
10
20
10
a
a
b
b
r
s
r x s:
Database System Concepts
A
B
C
D
E








1
1
1
1
2
2
2
2








10
10
20
10
10
10
20
10
a
a
b
b
a
a
b
b
3.32
©Silberschatz, Korth and Sudarshan
Cartesian-Product Operation
 Notation r x s
 Defined as:
r x s = {t q | t  r and q  s}
 Assume that attributes of r(R) and s(S) are disjoint. (That is,
R  S = ).
 If attributes of r(R) and s(S) are not disjoint, then renaming must
be used.
Database System Concepts
3.33
©Silberschatz, Korth and Sudarshan
Composition of Operations
 Can build expressions using multiple operations
 Example: A=C(r x s)
 rxs
 A=C(r x s)
Database System Concepts
A
B
C
D
E








1
1
1
1
2
2
2
2








10
10
20
10
10
10
20
10
a
a
b
b
a
a
b
b
A
B
C
D
E



1
2
2
 10
 20
 20
a
a
b
3.34
©Silberschatz, Korth and Sudarshan
Rename Operation
 Allows us to name, and therefore to refer to, the results of
relational-algebra expressions.
 Allows us to refer to a relation by more than one name.
Example:
 x (E)
returns the expression E under the name X
If a relational-algebra expression E has arity n, then
x (A1, A2, …, An) (E)
returns the result of expression E under the name X, and with the
attributes renamed to A1, A2, …., An.
Database System Concepts
3.35
©Silberschatz, Korth and Sudarshan
Banking Example
branch (branch-name, branch-city, assets)
customer (customer-name, customer-street, customer-only)
account (account-number, branch-name, balance)
loan (loan-number, branch-name, amount)
depositor (customer-name, account-number)
borrower (customer-name, loan-number)
Database System Concepts
3.36
©Silberschatz, Korth and Sudarshan
Example Queries
 Find all loans of over $1200
amount > 1200 (loan)
Find the loan number for each loan of an amount greater than
$1200
loan-number (amount > 1200 (loan))
Database System Concepts
3.37
©Silberschatz, Korth and Sudarshan
Example Queries
 Find the names of all customers who have a loan, an account, or
both, from the bank
customer-name (borrower)  customer-name (depositor)
Find the names of all customers who have a loan and an
account at bank.
customer-name (borrower)  customer-name (depositor)
Database System Concepts
3.38
©Silberschatz, Korth and Sudarshan
Example Queries
 Find the names of all customers who have a loan at the Perryridge
branch.
customer-name (branch-name=“Perryridge”
(borrower.loan-number = loan.loan-number(borrower x loan)))
 Find the names of all customers who have a loan at the
Perryridge branch but do not have an account at any branch of
the bank.
customer-name (branch-name = “Perryridge”
(borrower.loan-number = loan.loan-number(borrower x loan))) –
customer-name(depositor)
Database System Concepts
3.39
©Silberschatz, Korth and Sudarshan
Example Queries
 Find the names of all customers who have a loan at the Perryridge
branch.
Query 1
customer-name(branch-name = “Perryridge” (
borrower.loan-number = loan.loan-number(borrower x loan)))
 Query 2
customer-name(loan.loan-number = borrower.loan-number(
(branch-name = “Perryridge”(loan)) x borrower))
Database System Concepts
3.40
©Silberschatz, Korth and Sudarshan
Example Queries
Find the largest account balance
 Rename account relation as d
 The query is:
balance(account) - account.balance
(account.balance < d.balance (account x d (account)))
Database System Concepts
3.41
©Silberschatz, Korth and Sudarshan
Formal Definition
 A basic expression in the relational algebra consists of either one
of the following:
 A relation in the database
 A constant relation
 Let E1 and E2 be relational-algebra expressions; the following are
all relational-algebra expressions:
 E1  E2
 E1 - E2
 E1 x E2
 p (E1), P is a predicate on attributes in E1
 s(E1), S is a list consisting of some of the attributes in E1
  x (E1), x is the new name for the result of E1
Database System Concepts
3.42
©Silberschatz, Korth and Sudarshan
Additional Operations
We define additional operations that do not add any power to the
relational algebra, but that simplify common queries.
 Set intersection
 Natural join
 Division
 Assignment
Database System Concepts
3.43
©Silberschatz, Korth and Sudarshan
Set-Intersection Operation
 Notation: r  s
 Defined as:
 r  s ={ t | t  r and t  s }
 Assume:
 r, s have the same arity
 attributes of r and s are compatible
 Note: r  s = r - (r - s)
Database System Concepts
3.44
©Silberschatz, Korth and Sudarshan
Set-Intersection Operation - Example
 Relation r, s:
A
B



1
2
1
A


r
 rs
Database System Concepts
A
B

2
B
2
3
s
3.45
©Silberschatz, Korth and Sudarshan
Natural-Join Operation

Notation: r
s
 Let r and s be relations on schemas R and S respectively.
Then, r
s is a relation on schema R  S obtained as follows:
 Consider each pair of tuples tr from r and ts from s.
 If tr and ts have the same value on each of the attributes in R  S, add
a tuple t to the result, where
 t has the same value as t on r
r
 t has the same value as t
s on s
 Example:
R = (A, B, C, D)
S = (E, B, D)
 Result schema = (A, B, C, D, E)
 r
s is defined as:
r.A, r.B, r.C, r.D, s.E (r.B = s.B  r.D = s.D (r x s))
Database System Concepts
3.46
©Silberschatz, Korth and Sudarshan
Natural Join Operation – Example
 Relations r, s:
A
B
C
D
B
D
E





1
2
4
1
2





a
a
b
a
b
1
3
1
2
3
a
a
a
b
b





r
r
s
Database System Concepts
s
A
B
C
D
E





1
1
1
1
2





a
a
a
a
b





3.47
©Silberschatz, Korth and Sudarshan
Division Operation
rs
 Suited to queries that include the phrase “for all”.
 Let r and s be relations on schemas R and S respectively
where
 R = (A1, …, Am, B1, …, Bn)
 S = (B1, …, Bn)
The result of r  s is a relation on schema
R – S = (A1, …, Am)
r  s = { t | t   R-S(r)   u  s ( tu  r ) }
Database System Concepts
3.48
©Silberschatz, Korth and Sudarshan
Division Operation – Example
Relations r, s:
r  s:
A
A
B
B











1
2
3
1
1
1
3
4
6
1
2
1
2
s
r


Database System Concepts
3.49
©Silberschatz, Korth and Sudarshan
Another Division Example
Relations r, s:
A
B
C
D
E
D
E








a
a
a
a
a
a
a
a








a
a
b
a
b
a
b
b
1
1
1
1
3
1
1
1
a
b
1
1
s
r
r  s:
Database System Concepts
A
B
C


a
a


3.50
©Silberschatz, Korth and Sudarshan
Division Operation (Cont.)
 Property
 Let q – r  s
 Then q is the largest relation satisfying q x s  r
 Definition in terms of the basic algebra operation
Let r(R) and s(S) be relations, and let S  R
r  s = R-S (r) –R-S ( (R-S (r) x s) – R-S,S(r))
To see why
 R-S,S(r) simply reorders attributes of r
 R-S(R-S (r) x s) – R-S,S(r)) gives those tuples t in
R-S (r) such that for some tuple u  s, tu  r.
Database System Concepts
3.51
©Silberschatz, Korth and Sudarshan
Assignment Operation
 The assignment operation () provides a convenient way to
express complex queries.

Write query as a sequential program consisting of
 a series of assignments
 followed by an expression whose value is displayed as a result of
the query.
 Assignment must always be made to a temporary relation variable.
 Example: Write r  s as
temp1  R-S (r)
temp2  R-S ((temp1 x s) – R-S,S (r))
result = temp1 – temp2
 The result to the right of the  is assigned to the relation variable on
the left of the .
 May use variable in subsequent expressions.
Database System Concepts
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Example Queries
 Find all customers who have an account from at least the
“Downtown” and the Uptown” branches.
Query 1
CN(BN=“Downtown”(depositor
account)) 
CN(BN=“Uptown”(depositor
account))
where CN denotes customer-name and BN denotes
branch-name.
Query 2
customer-name, branch-name (depositor account)
 temp(branch-name) ({(“Downtown”), (“Uptown”)})
Database System Concepts
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©Silberschatz, Korth and Sudarshan
Example Queries
 Find all customers who have an account at all branches located
in Brooklyn city.
customer-name, branch-name (depositor account)
 branch-name (branch-city = “Brooklyn” (branch))
Database System Concepts
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©Silberschatz, Korth and Sudarshan
Extended Relational-Algebra-Operations
 Generalized Projection
 Outer Join
 Aggregate Functions
Database System Concepts
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©Silberschatz, Korth and Sudarshan
Generalized Projection
 Extends the projection operation by allowing arithmetic functions
to be used in the projection list.
 F1, F2, …, Fn(E)
 E is any relational-algebra expression
 Each of F1, F2, …, Fn are are arithmetic expressions involving
constants and attributes in the schema of E.
 Given relation credit-info(customer-name, limit, credit-balance),
find how much more each person can spend:
customer-name, limit – credit-balance (credit-info)
Database System Concepts
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Aggregate Functions and Operations
 Aggregation function takes a collection of values and returns a
single value as a result.
avg: average value
min: minimum value
max: maximum value
sum: sum of values
count: number of values
 Aggregate operation in relational algebra
G1, G2, …, Gn
g F1( A1), F2( A2),…, Fn( An) (E)
 E is any relational-algebra expression
 G1, G2 …, Gn is a list of attributes on which to group (can be empty)
 Each Fi is an aggregate function
 Each Ai is an attribute name
Database System Concepts
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Aggregate Operation – Example
 Relation r:
g sum(c) (r)
Database System Concepts
A
B
C








7
7
3
10
sum-C
27
3.58
©Silberschatz, Korth and Sudarshan
Aggregate Operation – Example
 Relation account grouped by branch-name:
branch-name account-number
Perryridge
Perryridge
Brighton
Brighton
Redwood
branch-name
g
A-102
A-201
A-217
A-215
A-222
sum(balance)
400
900
750
750
700
(account)
branch-name
Perryridge
Brighton
Redwood
Database System Concepts
balance
3.59
balance
1300
1500
700
©Silberschatz, Korth and Sudarshan
Aggregate Functions (Cont.)
 Result of aggregation does not have a name
 Can use rename operation to give it a name
 For convenience, we permit renaming as part of aggregate
operation
branch-name
Database System Concepts
g
sum(balance) as sum-balance (account)
3.60
©Silberschatz, Korth and Sudarshan
Outer Join
 An extension of the join operation that avoids loss of information.
 Computes the join and then adds tuples form one relation that
does not match tuples in the other relation to the result of the
join.
 Uses null values:
 null signifies that the value is unknown or does not exist
 All comparisons involving null are (roughly speaking) false by
definition.
 Will study precise meaning of comparisons with nulls later
Database System Concepts
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Outer Join – Example
 Relation loan
loan-number
branch-name
L-170
L-230
L-260
Downtown
Redwood
Perryridge
amount
3000
4000
1700
 Relation borrower
customer-name loan-number
Jones
Smith
Hayes
Database System Concepts
L-170
L-230
L-155
3.62
©Silberschatz, Korth and Sudarshan
Outer Join – Example
 Inner Join
loan
Borrower
loan-number
L-170
L-230
branch-name
Downtown
Redwood
amount
customer-name
3000
4000
Jones
Smith
amount
customer-name
 Left Outer Join
loan
Borrower
loan-number
L-170
L-230
L-260
Database System Concepts
branch-name
Downtown
Redwood
Perryridge
3000
4000
1700
3.63
Jones
Smith
null
©Silberschatz, Korth and Sudarshan
Outer Join – Example
 Right Outer Join
loan
borrower
loan-number
L-170
L-230
L-155
branch-name
Downtown
Redwood
null
amount
3000
4000
null
customer-name
Jones
Smith
Hayes
 Full Outer Join
loan
borrower
loan-number
L-170
L-230
L-260
L-155
Database System Concepts
branch-name
Downtown
Redwood
Perryridge
null
amount
3000
4000
1700
null
3.64
customer-name
Jones
Smith
null
Hayes
©Silberschatz, Korth and Sudarshan
Null Values
 It is possible for tuples to have a null value, denoted by null, for
some of their attributes
 null signifies an unknown value or that a value does not exist.
 The result of any arithmetic expression involving null is null.
 Aggregate functions simply ignore null values
 Is an arbitrary decision. Could have returned null as result instead.
 We follow the semantics of SQL in its handling of null values
 For duplicate elimination and grouping, null is treated like any
other value, and two nulls are assumed to be the same
 Alternative: assume each null is different from each other
 Both are arbitrary decisions, so we simply follow SQL
Database System Concepts
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Null Values
 Comparisons with null values return the special truth value
unknown
 If false was used instead of unknown, then
would not be equivalent to
not (A < 5)
A >= 5
 Three-valued logic using the truth value unknown:
 OR: (unknown or true)
= true,
(unknown or false)
= unknown
(unknown or unknown) = unknown
 AND: (true and unknown)
= unknown,
(false and unknown)
= false,
(unknown and unknown) = unknown
 NOT: (not unknown) = unknown
 In SQL “P is unknown” evaluates to true if predicate P evaluates
to unknown
 Result of select predicate is treated as false if it evaluates to
unknown
Database System Concepts
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Modification of the Database
 The content of the database may be modified using the following
operations:
 Deletion
 Insertion
 Updating
 All these operations are expressed using the assignment
operator.
Database System Concepts
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Deletion
 A delete request is expressed similarly to a query, except instead
of displaying tuples to the user, the selected tuples are removed
from the database.
 Can delete only whole tuples; cannot delete values on only
particular attributes
 A deletion is expressed in relational algebra by:
rr–E
where r is a relation and E is a relational algebra query.
Database System Concepts
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Deletion Examples
 Delete all account records in the Perryridge branch.
account  account – branch-name = “Perryridge” (account)
Delete all loan records with amount in the range of 0 to 50
loan  loan –  amount 0 and amount  50 (loan)
Delete all accounts at branches located in Needham.
r1   branch-city = “Needham” (account
branch)
r2  branch-name, account-number, balance (r1)
r3   customer-name, account-number (r2
depositor)
account  account – r2
depositor  depositor – r3
Database System Concepts
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Insertion
 To insert data into a relation, we either:
 specify a tuple to be inserted
 write a query whose result is a set of tuples to be inserted
 in relational algebra, an insertion is expressed by:
r r  E
where r is a relation and E is a relational algebra expression.
 The insertion of a single tuple is expressed by letting E be a
constant relation containing one tuple.
Database System Concepts
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Insertion Examples
 Insert information in the database specifying that Smith has
$1200 in account A-973 at the Perryridge branch.
account  account  {(“Perryridge”, A-973, 1200)}
depositor  depositor  {(“Smith”, A-973)}
 Provide as a gift for all loan customers in the Perryridge
branch, a $200 savings account. Let the loan number serve
as the account number for the new savings account.
r1  (branch-name = “Perryridge” (borrower
loan))
account  account  branch-name, account-number,200 (r1)
depositor  depositor  customer-name, loan-number(r1)
Database System Concepts
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©Silberschatz, Korth and Sudarshan
Updating
 A mechanism to change a value in a tuple without charging all
values in the tuple
 Use the generalized projection operator to do this task
r   F1, F2, …, FI, (r)
 Each Fi is either
 the ith attribute of r, if the ith attribute is not updated, or,
 if the attribute is to be updated Fi is an expression, involving only
constants and the attributes of r, which gives the new value for the
attribute
Database System Concepts
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©Silberschatz, Korth and Sudarshan
Update Examples
 Make interest payments by increasing all balances by 5 percent.
account   AN, BN, BAL * 1.05 (account)
where AN, BN and BAL stand for account-number, branch-name
and balance, respectively.
 Pay all accounts with balances over $10,000 6 percent interest
and pay all others 5 percent
account 
Database System Concepts
 AN, BN, BAL * 1.06 ( BAL  10000 (account))
 AN, BN, BAL * 1.05 (BAL  10000 (account))
3.73
©Silberschatz, Korth and Sudarshan
Views
 In some cases, it is not desirable for all users to see the entire
logical model (i.e., all the actual relations stored in the database.)
 Consider a person who needs to know a customer’s loan number
but has no need to see the loan amount. This person should see
a relation described, in the relational algebra, by
customer-name, loan-number (borrower
loan)
 Any relation that is not of the conceptual model but is made
visible to a user as a “virtual relation” is called a view.
Database System Concepts
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View Definition
 A view is defined using the create view statement which has the
form
create view v as <query expression
where <query expression> is any legal relational algebra query
expression. The view name is represented by v.
 Once a view is defined, the view name can be used to refer to
the virtual relation that the view generates.
 View definition is not the same as creating a new relation by
evaluating the query expression
 Rather, a view definition causes the saving of an expression; the
expression is substituted into queries using the view.
Database System Concepts
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©Silberschatz, Korth and Sudarshan
View Examples
 Consider the view (named all-customer) consisting of branches
and their customers.
create view all-customer as
branch-name, customer-name (depositor
account)
 branch-name, customer-name (borrower
loan)
 We can find all customers of the Perryridge branch by writing:
branch-name
(branch-name = “Perryridge” (all-customer))
Database System Concepts
3.76
©Silberschatz, Korth and Sudarshan
Updates Through View
 Database modifications expressed as views must be translated
to modifications of the actual relations in the database.
 Consider the person who needs to see all loan data in the loan
relation except amount. The view given to the person, branchloan, is defined as:
create view branch-loan as
branch-name, loan-number (loan)
 Since we allow a view name to appear wherever a relation name
is allowed, the person may write:
branch-loan  branch-loan  {(“Perryridge”, L-37)}
Database System Concepts
3.77
©Silberschatz, Korth and Sudarshan
Updates Through Views (Cont.)
 The previous insertion must be represented by an insertion into the
actual relation loan from which the view branch-loan is constructed.
 An insertion into loan requires a value for amount. The insertion
can be dealt with by either.
 rejecting the insertion and returning an error message to the user.
 inserting a tuple (“L-37”, “Perryridge”, null) into the loan relation
 Some updates through views are impossible to translate into
database relation updates
 create view v as branch-name = “Perryridge” (account))
v  v  (L-99, Downtown, 23)
 Others cannot be translated uniquely
 all-customer  all-customer  {(“Perryridge”, “John”)}
 Have to choose loan or account, and
create a new loan/account number!
Database System Concepts
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©Silberschatz, Korth and Sudarshan
Views Defined Using Other Views
 One view may be used in the expression defining another view
 A view relation v1 is said to depend directly on a view relation v2
if v2 is used in the expression defining v1
 A view relation v1 is said to depend on view relation v2 if either v1
depends directly to v2 or there is a path of dependencies from
v1 to v2
 A view relation v is said to be recursive if it depends on itself.
Database System Concepts
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©Silberschatz, Korth and Sudarshan
View Expansion
 A way to define the meaning of views defined in terms of other
views.
 Let view v1 be defined by an expression e1 that may itself contain
uses of view relations.
 View expansion of an expression repeats the following
replacement step:
repeat
Find any view relation vi in e1
Replace the view relation vi by the expression defining vi
until no more view relations are present in e1
 As long as the view definitions are not recursive, this loop will
terminate
Database System Concepts
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Tuple Relational Calculus
 A nonprocedural query language, where each query is of the form
{t | P (t) }
 It is the set of all tuples t such that predicate P is true for t
 t is a tuple variable, t[A] denotes the value of tuple t on attribute A
 t  r denotes that tuple t is in relation r
 P is a formula similar to that of the predicate calculus
Database System Concepts
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©Silberschatz, Korth and Sudarshan
Predicate Calculus Formula
1. Set of attributes and constants
2. Set of comparison operators: (e.g., , , , , , )
3. Set of connectives: and (), or (v)‚ not ()
4. Implication (): x  y, if x if true, then y is true
x  y x v y
5. Set of quantifiers:

 t  r (Q(t))  ”there exists” a tuple in t in relation r
such that predicate Q(t) is true

t r (Q(t)) Q is true “for all” tuples t in relation r
Database System Concepts
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©Silberschatz, Korth and Sudarshan
Banking Example
 branch (branch-name, branch-city, assets)
 customer (customer-name, customer-street, customer-city)
 account (account-number, branch-name, balance)
 loan (loan-number, branch-name, amount)
 depositor (customer-name, account-number)
 borrower (customer-name, loan-number)
Database System Concepts
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©Silberschatz, Korth and Sudarshan
Example Queries
 Find the loan-number, branch-name, and amount for loans of
over $1200
{t | t  loan  t [amount]  1200}
Find the loan number for each loan of an amount greater than $1200
{t |  s loan (t[loan-number] = s[loan-number]  s [amount]  1200)}
Notice that a relation on schema [loan-number] is implicitly defined
by the query
Database System Concepts
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©Silberschatz, Korth and Sudarshan
Example Queries
 Find the names of all customers having a loan, an account, or
both at the bank
{t | s  borrower( t[customer-name] = s[customer-name])
 u  depositor( t[customer-name] = u[customer-name])
 Find the names of all customers who have a loan and an account
at the bank
{t | s  borrower( t[customer-name] = s[customer-name])
 u  depositor( t[customer-name] = u[customer-name])
Database System Concepts
3.85
©Silberschatz, Korth and Sudarshan
Example Queries
 Find the names of all customers having a loan at the Perryridge
branch
{t | s  borrower(t[customer-name] = s[customer-name]
 u  loan(u[branch-name] = “Perryridge”
 u[loan-number] = s[loan-number]))}
 Find the names of all customers who have a loan at the
Perryridge branch, but no account at any branch of the bank
{t | s  borrower( t[customer-name] = s[customer-name]
 u  loan(u[branch-name] = “Perryridge”
 u[loan-number] = s[loan-number]))
 not v  depositor (v[customer-name] =
t[customer-name]) }
Database System Concepts
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©Silberschatz, Korth and Sudarshan
Example Queries
 Find the names of all customers having a loan from the
Perryridge branch, and the cities they live in
{t | s  loan(s[branch-name] = “Perryridge”
 u  borrower (u[loan-number] = s[loan-number]
 t [customer-name] = u[customer-name])
  v  customer (u[customer-name] = v[customer-name]
 t[customer-city] = v[customer-city])))}
Database System Concepts
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©Silberschatz, Korth and Sudarshan
Example Queries
 Find the names of all customers who have an account at all
branches located in Brooklyn:
{t |  c  customer (t[customer.name] = c[customer-name]) 
 s  branch(s[branch-city] = “Brooklyn” 
 u  account ( s[branch-name] = u[branch-name]
  s  depositor ( t[customer-name] = s[customer-name]
 s[account-number] = u[account-number] )) )}
Database System Concepts
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Safety of Expressions
 It is possible to write tuple calculus expressions that generate
infinite relations.
 For example, {t |  t r} results in an infinite relation if the
domain of any attribute of relation r is infinite
 To guard against the problem, we restrict the set of allowable
expressions to safe expressions.
 An expression {t | P(t)} in the tuple relational calculus is safe if
every component of t appears in one of the relations, tuples, or
constants that appear in P
 NOTE: this is more than just a syntax condition.
 E.g. { t | t[A]=5
 true } is not safe --- it defines an infinite set with
attribute values that do not appear in any relation or tuples or
constants in P.
Database System Concepts
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©Silberschatz, Korth and Sudarshan
Domain Relational Calculus
 A nonprocedural query language equivalent in power to the tuple
relational calculus
 Each query is an expression of the form:
{  x1, x2, …, xn  | P(x1, x2, …, xn)}
 x1, x2, …, xn represent domain variables
 P represents a formula similar to that of the predicate calculus
Database System Concepts
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©Silberschatz, Korth and Sudarshan
Example Queries
 Find the loan-number, branch-name, and amount for loans of over
$1200
{ l, b, a  |  l, b, a   loan  a > 1200}
 Find the names of all customers who have a loan of over $1200
{ c  |  l, b, a ( c, l   borrower   l, b, a   loan  a > 1200)}
 Find the names of all customers who have a loan from the
Perryridge branch and the loan amount:
{ c, a  |  l ( c, l   borrower  b( l, b, a   loan 
b = “Perryridge”))}
or { c, a  |  l ( c, l   borrower   l, “Perryridge”, a   loan)}
Database System Concepts
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©Silberschatz, Korth and Sudarshan
Example Queries
 Find the names of all customers having a loan, an account, or
both at the Perryridge branch:
{ c  |  l ({ c, l   borrower
  b,a( l, b, a   loan  b = “Perryridge”))
  a( c, a   depositor
  b,n( a, b, n   account  b = “Perryridge”))}
 Find the names of all customers who have an account at all
branches located in Brooklyn:
{ c  |  s, n ( c, s, n   customer) 
 x,y,z( x, y, z   branch  y = “Brooklyn”) 
 a,b( x, y, z   account   c,a   depositor)}
Database System Concepts
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©Silberschatz, Korth and Sudarshan
Safety of Expressions
{  x1, x2, …, xn  | P(x1, x2, …, xn)}
is safe if all of the following hold:
1.All values that appear in tuples of the expression are values
from dom(P) (that is, the values appear either in P or in a tuple
of a relation mentioned in P).
2.For every “there exists” subformula of the form  x (P1(x)), the
subformula is true if and only if there is a value of x in dom(P1)
such that P1(x) is true.
3. For every “for all” subformula of the form x (P1 (x)), the
subformula is true if and only if P1(x) is true for all values x
from dom (P1).
Database System Concepts
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©Silberschatz, Korth and Sudarshan
End of Chapter 3
Result of  branch-name = “Perryridge” (loan)
Database System Concepts
3.95
©Silberschatz, Korth and Sudarshan
Loan Number and the Amount of the Loan
Database System Concepts
3.96
©Silberschatz, Korth and Sudarshan
Names of All Customers Who Have
Either a Loan or an Account
Database System Concepts
3.97
©Silberschatz, Korth and Sudarshan
Customers With An Account But No Loan
Database System Concepts
3.98
©Silberschatz, Korth and Sudarshan
Result of borrower  loan
Database System Concepts
3.99
©Silberschatz, Korth and Sudarshan
Result of  branch-name = “Perryridge” (borrower  loan)
Database System Concepts
3.100
©Silberschatz, Korth and Sudarshan
Result of customer-name
Database System Concepts
3.101
©Silberschatz, Korth and Sudarshan
Result of the Subexpression
Database System Concepts
3.102
©Silberschatz, Korth and Sudarshan
Largest Account Balance in the Bank
Database System Concepts
3.103
©Silberschatz, Korth and Sudarshan
Customers Who Live on the Same Street and In the
Same City as Smith
Database System Concepts
3.104
©Silberschatz, Korth and Sudarshan
Customers With Both an Account and a Loan
at the Bank
Database System Concepts
3.105
©Silberschatz, Korth and Sudarshan
Result of customer-name, loan-number, amount
(borrower
loan)
Database System Concepts
3.106
©Silberschatz, Korth and Sudarshan
Result of branch-name(customer-city =
account
depositor))
“Harrison”(customer
Database System Concepts
3.107
©Silberschatz, Korth and Sudarshan
Result of branch-name(branch-city =
“Brooklyn”(branch))
Database System Concepts
3.108
©Silberschatz, Korth and Sudarshan
Result of customer-name, branch-name(depositor
Database System Concepts
3.109
account)
©Silberschatz, Korth and Sudarshan
The credit-info Relation
Database System Concepts
3.110
©Silberschatz, Korth and Sudarshan
Result of customer-name, (limit – credit-balance) as
credit-available(credit-info).
Database System Concepts
3.111
©Silberschatz, Korth and Sudarshan
The pt-works Relation
Database System Concepts
3.112
©Silberschatz, Korth and Sudarshan
The pt-works Relation After Grouping
Database System Concepts
3.113
©Silberschatz, Korth and Sudarshan
Result of branch-name  sum(salary) (pt-works)
Database System Concepts
3.114
©Silberschatz, Korth and Sudarshan
Result of branch-name  sum salary, max(salary) as
max-salary (pt-works)
Database System Concepts
3.115
©Silberschatz, Korth and Sudarshan
The employee and ft-works Relations
Database System Concepts
3.116
©Silberschatz, Korth and Sudarshan
The Result of employee
Database System Concepts
3.117
ft-works
©Silberschatz, Korth and Sudarshan
The Result of employee
Database System Concepts
3.118
ft-works
©Silberschatz, Korth and Sudarshan
Result of employee
Database System Concepts
3.119
ft-works
©Silberschatz, Korth and Sudarshan
Result of employee
Database System Concepts
3.120
ft-works
©Silberschatz, Korth and Sudarshan
Tuples Inserted Into loan and borrower
Database System Concepts
3.121
©Silberschatz, Korth and Sudarshan
Names of All Customers Who Have a
Loan at the Perryridge Branch
Database System Concepts
3.122
©Silberschatz, Korth and Sudarshan
E-R Diagram
Database System Concepts
3.123
©Silberschatz, Korth and Sudarshan
The branch Relation
Database System Concepts
3.124
©Silberschatz, Korth and Sudarshan
The loan Relation
Database System Concepts
3.125
©Silberschatz, Korth and Sudarshan
The borrower Relation
Database System Concepts
3.126
©Silberschatz, Korth and Sudarshan